Press forming is one of the typical metal processing methods that produces a part with desired shape by the application of pressure to a metal sheet such as a steel sheet interposed between a pair of mold halves so that the metal sheet is formed in conformity with the shape of the mold. This technique is used in a wide range of fields such as manufacturing of automobile parts, machinery parts, building parts and home electric appliances.
In recent years, high strength steel sheets are increasingly used particularly in the press forming of automobile parts. However, workpieces having higher strength show lower press formability. This problem is addressed by improving both strength and elongation and thereby enhancing the mechanical properties of metal sheets themselves. A few of such metal sheets are dual phase steel sheets including hard and soft phases in the metal microstructure, and TRIP (transformation induced plasticity) steel sheets utilizing retained austenite. Such an approach is based on the fact that press formability is correlated to the elongation of metal sheets.
In general, the press formability of metal sheets is represented by a forming limit diagram. A forming limit diagram shows a critical value of strain at or immediately before the occurrence of fracture in metal sheets under the application of various types of biaxial stress to the metal sheets. In order to enhance the accuracy of the measurement or prediction of this forming limit, positive attempts have been made involving studies of the influences of various properties of materials (see, for example, Non Patent Literature 1).
Further, a technique has been developed which simulates press forming by a finite element method with use of a forming limit diagram to determine the forming conditions that will not cause fracture of metal sheets (see, for example, Patent Literature 1).